A variety of corals form an outcrop on Flynn Reef, part of the Great Barrier Reef near Cairns, Queensland, Australia. (Credit: Toby Hudson/Creative Commons Attribution-Share Alike 3.0 Unported license.)

Corals are colonies comprised of countless millions of genetically identical animals, known as polyps. Each polyp secretes a hard chamber, or skeleton, made from calcium carbonate that surrounds and protects its soft translucent body. The specific shape of this skeleton is characteristic of the coral species, and the brown colours assumed by those corals are provided by their specific endosymbionts. Thus, when the coral expels its zooxanthellae endosymbionts, it loses its colour, leaving the white skeleton behind. The coral colony, which is still alive (at least for awhile), is thus "bleached".

Hard corals are the most perfect of landlords: they provide safe little apartments stocked with food that fulfil almost every need that a living thing could wish for, and these apartments rise up towards the sun so their microscopic inhabitants can "pay their rent" by doing the one thing that their coral landlords cannot do: they harness the power contained within sunlight to create usable energy through photosynthesis. This energy is shared with their hosts. A mutually beneficial relationship, such as this, is known as symbiosis, and because these microbes live inside individual coral polyps, they are essentially beneficial roommates, known as endosymbionts.

The corals' endosymbionts are known colloquially as "zooxanthellae". Because zooxanthellae are photosynthetic, they are often referred to as algae, although they actually comprise a diverse group of distinct species, many of which are not even close relatives. These single-celled yellowish-brown organisms are classified together into the same taxonomic group (genus: Symbiodinium) because they look alike and do similar things, but most species are specialised for life within the cells of particular sorts of marine invertebrates. For example, the zooxanthellae pictured below are endosymbionts of the jellyfish, Cassiopea xamachana:

Light and confocal images of Symbiodinium cells in hospite (living in a host cell) within scyphistomae of the jellyfish Cassiopea xamachana. This animal requires infection by these algae to complete its life cycle. The chloroplast imaged in 3-D is highly reticulated and distributed around the cell’s periphery. (Credit: AllisonMLewis/CC BY-SA 4.0.)

Just as different zooxanthellae species tend to associate with particular hosts, each species is sensitive to different combinations, and sorts, of stressful conditions (doi:10.1371/journal.pone.0114321 & doi:10.1074/mcp.M114.043125). Likewise, corals also differ in the sorts of stressors they are most sensitive to. For example, branching corals, such as the Acropora coral pictured above, are more sensitive to temperature fluctuations than are compact corals.

Although most evidence reveals that elevated water temperature is the primary cause of mass bleaching events (doi:10.1086/286181), there are a variety of other stressful conditions that also damage corals: excessively cool water temperatures; variations in pH or salinity; pollution; increased solar radiation; weather events, such as cyclones; or disease. Unfortunately, the effects of increasingly affluent human populations and of global climate change have made stressful conditions progressively more diverse, more frequent and longer in duration.

When corals become stressed, they struggle to provide enough nutrients to support their endosymbionts' photosynthetic activities (doi:10.1002/bies.200900182). Thus, the coral polyps' formerly helpful roommates are transformed into overly-demanding houseguests. This upsets the delicate balance between the needs of the coral and of its microbial endosymbionts, and thus, both host and guest become ill. Eventually the situation becomes dire for both organisms and, like a landlord who evicts rowdy roommates, coral polyps expel their ailing or dying zooxanthellae endosymbionts, leaving the coral behind.

Zooxanthellae cells provide corals with pigmentation. On the left is a healthy stony coral. On the right is a stony coral that has lost its zooxanthellae cells and has taken on a bleached appearance. If a coral polyp is without zooxanthellae cells for a long period of time, it will most likely die. (Credit: NOAA/Public Domain.)

Expulsion of zooxanthellae increases the chance that corals will survive stressful events in the short term. Since zooxanthellae can swim, surviving corals can attract new endosymbionts, possibly different species, after the stressor has ended (doi:10.2307/1543614). But bleached corals become progressively weaker as time passes and, in the absence of their photosynthetic endosymbionts, they slowly starve. Eventually, they too, will die if they cannot attract new zooxanthellae.